In the state of the art, the concept of active sound damping means a measure whereby a noise which is phase-shifted by 180.degree., is superimposed on the noise to be dampened. Such arrangements, which operate according to the sound wave suppression principle, are generally designed so that a device acquires the noise to be dampened electroacoustically, and a signal processing device transfers it to a signal that is 180.degree. phase-shifted with respect to the detected signal, before using the phase-shifted signal to control an electroacoustical converter.
If this principle is used for example to treat exhaust gas flows, such as occur in the exhaust equipment of combustion machines, in addition to the sound damping it must be ensured that the gas flow in the exhaust installation itself is not impaired by the acoustic radiation of the "anti-sound". To comply with this requirement, according to the state of the art, at least one acoustic converter in the form of a loudspeaker is so connected to the exhaust system, that a side of the diaphragm instead comes in direct contact with the exhaust gas flowing by. Such an arrangement is shown in greater detail in FIG. 2. This illustration of a principle shows a gas exhaust tube 10, in which exhaust gas (indicated by dots) coming from the combustion machine (not illustrated) flows in the direction of the arrow. Two opposing openings 11 are located on both sides of the gas exhaust tube 10. A cone loudspeaker 12 is placed into each of these openings 11 in such a way, that the diaphragms 13 of both loudspeakers 12 face each other, therefore the diaphragm sides 14 facing each other come in direct contact with the exhaust gases. The latter is made clear in that the exhaust gas also fills the areas B of loudspeaker 12, formed by the diaphragm cones 13. The rearward part of loudspeaker 12, and thus the part enclosing the magnet system 15, is surrounded by a housing 16, which encapsulates the loudspeakers with respect to the environment.
Although such an arrangement, whose loudspeaker 12 charges the sound in the gas exhaust tube 10 with sound signals that are phase-shifted by 180.degree., produces good sound cancellation in the gas exhaust tube, it is however considered a disadvantage that the loudspeaker diaphragms 13 of such an arrangement come in direct contact with the exhaust gas and the gas exhaust tube. This is because the exhaust gases are thermally and chemically charged, and these charges change the acoustic characteristics of the loudspeakers during operation on the one hand, and considerably shorten the useful life of the diaphragms 13 and their supports (not illustrated) on the other. Even the use of improved materials and costly adhesives for building suitable loudspeakers only solves the useful life problem insufficiently, as was proven by the applicant's tests, because the use of materials that are able to withstand temperature and chemicals is only conditionally possible, for acoustical reasons.
Another arrangement for active sound damping is known from the not yet published application DE 43 17 403.5. According to this arrangement, the "anti-sound" is produced in a loudspeaker box constructed according to the double chamber principle, and a bass reflex tube located in the front chamber of the box joins it to the gas exhaust line outside of the box. Although with such an arrangement the thermal and chemical charge of the loudspeaker diaphragm is small, such an arrangement requires exceptionally high sound pressures, so that continuous output of about 160 watts from the loudspeaker(s) is no rarity. In addition, the sound cancellation produced with this arrangement cannot be seen as optimum, because this arrangement produces a dipole radiator which exhibits interference phenomena in space, i.e. outside of the gas exhaust installation, and therefore does not produce sound cancellation in each space. Furthermore, the poor sound cancellation of the arrangement known from to DE 43 17 403.5 can also be attributed to the fact that the flow velocity inside the gas exhaust tube is significantly greater than the air moved by the diaphragm.
An arrangement is also known from PCT/GB92/01594, wherein the loudspeaker is located in the dividing wall of a box operating according to the double chamber principle. In this arrangement as well, the respective loudspeaker is located very far from the area where the cancellation of the sound waves, contained in the exhaust gas flow, takes place through the anti-sound produced by the loudspeaker. In other words, because of this large distance, the space that exists in publication PCT/GB92/01594 for guiding the sound waves emitted by the loudspeaker to the cancellation area, has the effect of a bass reflex tube shown in DE 43 17 403.5 so that the loudspeakers used in the arrangement according to PCT/GB92/01594 also require a high continuous output.
It is therefore the task of the invention to present an active sound damping system which avoids the disadvantages of the state of the art.